// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "ui/gfx/skbitmap_operations.h"

#include <algorithm>
#include <stddef.h>
#include <stdint.h>
#include <string.h>

#include "base/logging.h"
#include "skia/ext/refptr.h"
#include "third_party/skia/include/core/SkBitmap.h"
#include "third_party/skia/include/core/SkCanvas.h"
#include "third_party/skia/include/core/SkColorFilter.h"
#include "third_party/skia/include/core/SkColorPriv.h"
#include "third_party/skia/include/core/SkUnPreMultiply.h"
#include "third_party/skia/include/effects/SkBlurImageFilter.h"
#include "ui/gfx/geometry/insets.h"
#include "ui/gfx/geometry/point.h"
#include "ui/gfx/geometry/size.h"

// static
SkBitmap SkBitmapOperations::CreateInvertedBitmap(const SkBitmap& image)
{
    DCHECK(image.colorType() == kN32_SkColorType);

    SkAutoLockPixels lock_image(image);

    SkBitmap inverted;
    inverted.allocN32Pixels(image.width(), image.height());

    for (int y = 0; y < image.height(); ++y) {
        uint32_t* image_row = image.getAddr32(0, y);
        uint32_t* dst_row = inverted.getAddr32(0, y);

        for (int x = 0; x < image.width(); ++x) {
            uint32_t image_pixel = image_row[x];
            dst_row[x] = (image_pixel & 0xFF000000) | (0x00FFFFFF - (image_pixel & 0x00FFFFFF));
        }
    }

    return inverted;
}

// static
SkBitmap SkBitmapOperations::CreateBlendedBitmap(const SkBitmap& first,
    const SkBitmap& second,
    double alpha)
{
    DCHECK((alpha >= 0) && (alpha <= 1));
    DCHECK(first.width() == second.width());
    DCHECK(first.height() == second.height());
    DCHECK(first.bytesPerPixel() == second.bytesPerPixel());
    DCHECK(first.colorType() == kN32_SkColorType);

    // Optimize for case where we won't need to blend anything.
    static const double alpha_min = 1.0 / 255;
    static const double alpha_max = 254.0 / 255;
    if (alpha < alpha_min)
        return first;
    else if (alpha > alpha_max)
        return second;

    SkAutoLockPixels lock_first(first);
    SkAutoLockPixels lock_second(second);

    SkBitmap blended;
    blended.allocN32Pixels(first.width(), first.height());

    double first_alpha = 1 - alpha;

    for (int y = 0; y < first.height(); ++y) {
        uint32_t* first_row = first.getAddr32(0, y);
        uint32_t* second_row = second.getAddr32(0, y);
        uint32_t* dst_row = blended.getAddr32(0, y);

        for (int x = 0; x < first.width(); ++x) {
            uint32_t first_pixel = first_row[x];
            uint32_t second_pixel = second_row[x];

            int a = static_cast<int>((SkColorGetA(first_pixel) * first_alpha) + (SkColorGetA(second_pixel) * alpha));
            int r = static_cast<int>((SkColorGetR(first_pixel) * first_alpha) + (SkColorGetR(second_pixel) * alpha));
            int g = static_cast<int>((SkColorGetG(first_pixel) * first_alpha) + (SkColorGetG(second_pixel) * alpha));
            int b = static_cast<int>((SkColorGetB(first_pixel) * first_alpha) + (SkColorGetB(second_pixel) * alpha));

            dst_row[x] = SkColorSetARGB(a, r, g, b);
        }
    }

    return blended;
}

// static
SkBitmap SkBitmapOperations::CreateMaskedBitmap(const SkBitmap& rgb,
    const SkBitmap& alpha)
{
    DCHECK(rgb.width() == alpha.width());
    DCHECK(rgb.height() == alpha.height());
    DCHECK(rgb.bytesPerPixel() == alpha.bytesPerPixel());
    DCHECK(rgb.colorType() == kN32_SkColorType);
    DCHECK(alpha.colorType() == kN32_SkColorType);

    SkBitmap masked;
    masked.allocN32Pixels(rgb.width(), rgb.height());

    SkAutoLockPixels lock_rgb(rgb);
    SkAutoLockPixels lock_alpha(alpha);
    SkAutoLockPixels lock_masked(masked);

    for (int y = 0; y < masked.height(); ++y) {
        uint32_t* rgb_row = rgb.getAddr32(0, y);
        uint32_t* alpha_row = alpha.getAddr32(0, y);
        uint32_t* dst_row = masked.getAddr32(0, y);

        for (int x = 0; x < masked.width(); ++x) {
            unsigned alpha = SkGetPackedA32(alpha_row[x]);
            unsigned scale = SkAlpha255To256(alpha);
            dst_row[x] = SkAlphaMulQ(rgb_row[x], scale);
        }
    }

    return masked;
}

// static
SkBitmap SkBitmapOperations::CreateButtonBackground(SkColor color,
    const SkBitmap& image,
    const SkBitmap& mask)
{
    DCHECK(image.colorType() == kN32_SkColorType);
    DCHECK(mask.colorType() == kN32_SkColorType);

    SkBitmap background;
    background.allocN32Pixels(mask.width(), mask.height());

    double bg_a = SkColorGetA(color);
    double bg_r = SkColorGetR(color);
    double bg_g = SkColorGetG(color);
    double bg_b = SkColorGetB(color);

    SkAutoLockPixels lock_mask(mask);
    SkAutoLockPixels lock_image(image);
    SkAutoLockPixels lock_background(background);

    for (int y = 0; y < mask.height(); ++y) {
        uint32_t* dst_row = background.getAddr32(0, y);
        uint32_t* image_row = image.getAddr32(0, y % image.height());
        uint32_t* mask_row = mask.getAddr32(0, y);

        for (int x = 0; x < mask.width(); ++x) {
            uint32_t image_pixel = image_row[x % image.width()];

            double img_a = SkColorGetA(image_pixel);
            double img_r = SkColorGetR(image_pixel);
            double img_g = SkColorGetG(image_pixel);
            double img_b = SkColorGetB(image_pixel);

            double img_alpha = static_cast<double>(img_a) / 255.0;
            double img_inv = 1 - img_alpha;

            double mask_a = static_cast<double>(SkColorGetA(mask_row[x])) / 255.0;

            dst_row[x] = SkColorSetARGB(
                static_cast<int>(std::min(255.0, bg_a + img_a) * mask_a),
                static_cast<int>(((bg_r * img_inv) + (img_r * img_alpha)) * mask_a),
                static_cast<int>(((bg_g * img_inv) + (img_g * img_alpha)) * mask_a),
                static_cast<int>(((bg_b * img_inv) + (img_b * img_alpha)) * mask_a));
        }
    }

    return background;
}

namespace {
namespace HSLShift {

    // TODO(viettrungluu): Some things have yet to be optimized at all.

    // Notes on and conventions used in the following code
    //
    // Conventions:
    //  - R, G, B, A = obvious; as variables: |r|, |g|, |b|, |a| (see also below)
    //  - H, S, L = obvious; as variables: |h|, |s|, |l| (see also below)
    //  - variables derived from S, L shift parameters: |sdec| and |sinc| for S
    //    increase and decrease factors, |ldec| and |linc| for L (see also below)
    //
    // To try to optimize HSL shifts, we do several things:
    //  - Avoid unpremultiplying (then processing) then premultiplying. This means
    //    that R, G, B values (and also L, but not H and S) should be treated as
    //    having a range of 0..A (where A is alpha).
    //  - Do things in integer/fixed-point. This avoids costly conversions between
    //    floating-point and integer, though I should study the tradeoff more
    //    carefully (presumably, at some point of processing complexity, converting
    //    and processing using simpler floating-point code will begin to win in
    //    performance). Also to be studied is the speed/type of floating point
    //    conversions; see, e.g., <http://www.stereopsis.com/sree/fpu2006.html>.
    //
    // Conventions for fixed-point arithmetic
    //  - Each function has a constant denominator (called |den|, which should be a
    //    power of 2), appropriate for the computations done in that function.
    //  - A value |x| is then typically represented by a numerator, named |x_num|,
    //    so that its actual value is |x_num / den| (casting to floating-point
    //    before division).
    //  - To obtain |x_num| from |x|, simply multiply by |den|, i.e., |x_num = x *
    //    den| (casting appropriately).
    //  - When necessary, a value |x| may also be represented as a numerator over
    //    the denominator squared (set |den2 = den * den|). In such a case, the
    //    corresponding variable is called |x_num2| (so that its actual value is
    //    |x_num^2 / den2|.
    //  - The representation of the product of |x| and |y| is be called |x_y_num| if
    //    |x * y == x_y_num / den|, and |xy_num2| if |x * y == x_y_num2 / den2|. In
    //    the latter case, notice that one can calculate |x_y_num2 = x_num * y_num|.

    // Routine used to process a line; typically specialized for specific kinds of
    // HSL shifts (to optimize).
    typedef void (*LineProcessor)(const color_utils::HSL&,
        const SkPMColor*,
        SkPMColor*,
        int width);

    enum OperationOnH { kOpHNone = 0,
        kOpHShift,
        kNumHOps };
    enum OperationOnS { kOpSNone = 0,
        kOpSDec,
        kOpSInc,
        kNumSOps };
    enum OperationOnL { kOpLNone = 0,
        kOpLDec,
        kOpLInc,
        kNumLOps };

    // Epsilon used to judge when shift values are close enough to various critical
    // values (typically 0.5, which yields a no-op for S and L shifts. 1/256 should
    // be small enough, but let's play it safe>
    const double epsilon = 0.0005;

    // Line processor: default/universal (i.e., old-school).
    void LineProcDefault(const color_utils::HSL& hsl_shift,
        const SkPMColor* in,
        SkPMColor* out,
        int width)
    {
        for (int x = 0; x < width; x++) {
            out[x] = SkPreMultiplyColor(color_utils::HSLShift(
                SkUnPreMultiply::PMColorToColor(in[x]), hsl_shift));
        }
    }

    // Line processor: no-op (i.e., copy).
    void LineProcCopy(const color_utils::HSL& hsl_shift,
        const SkPMColor* in,
        SkPMColor* out,
        int width)
    {
        DCHECK(hsl_shift.h < 0);
        DCHECK(hsl_shift.s < 0 || fabs(hsl_shift.s - 0.5) < HSLShift::epsilon);
        DCHECK(hsl_shift.l < 0 || fabs(hsl_shift.l - 0.5) < HSLShift::epsilon);
        memcpy(out, in, static_cast<size_t>(width) * sizeof(out[0]));
    }

    // Line processor: H no-op, S no-op, L decrease.
    void LineProcHnopSnopLdec(const color_utils::HSL& hsl_shift,
        const SkPMColor* in,
        SkPMColor* out,
        int width)
    {
        const uint32_t den = 65536;

        DCHECK(hsl_shift.h < 0);
        DCHECK(hsl_shift.s < 0 || fabs(hsl_shift.s - 0.5) < HSLShift::epsilon);
        DCHECK(hsl_shift.l <= 0.5 - HSLShift::epsilon && hsl_shift.l >= 0);

        uint32_t ldec_num = static_cast<uint32_t>(hsl_shift.l * 2 * den);
        for (int x = 0; x < width; x++) {
            uint32_t a = SkGetPackedA32(in[x]);
            uint32_t r = SkGetPackedR32(in[x]);
            uint32_t g = SkGetPackedG32(in[x]);
            uint32_t b = SkGetPackedB32(in[x]);
            r = r * ldec_num / den;
            g = g * ldec_num / den;
            b = b * ldec_num / den;
            out[x] = SkPackARGB32(a, r, g, b);
        }
    }

    // Line processor: H no-op, S no-op, L increase.
    void LineProcHnopSnopLinc(const color_utils::HSL& hsl_shift,
        const SkPMColor* in,
        SkPMColor* out,
        int width)
    {
        const uint32_t den = 65536;

        DCHECK(hsl_shift.h < 0);
        DCHECK(hsl_shift.s < 0 || fabs(hsl_shift.s - 0.5) < HSLShift::epsilon);
        DCHECK(hsl_shift.l >= 0.5 + HSLShift::epsilon && hsl_shift.l <= 1);

        uint32_t linc_num = static_cast<uint32_t>((hsl_shift.l - 0.5) * 2 * den);
        for (int x = 0; x < width; x++) {
            uint32_t a = SkGetPackedA32(in[x]);
            uint32_t r = SkGetPackedR32(in[x]);
            uint32_t g = SkGetPackedG32(in[x]);
            uint32_t b = SkGetPackedB32(in[x]);
            r += (a - r) * linc_num / den;
            g += (a - g) * linc_num / den;
            b += (a - b) * linc_num / den;
            out[x] = SkPackARGB32(a, r, g, b);
        }
    }

    // Saturation changes modifications in RGB
    //
    // (Note that as a further complication, the values we deal in are
    // premultiplied, so R/G/B values must be in the range 0..A. For mathematical
    // purposes, one may as well use r=R/A, g=G/A, b=B/A. Without loss of
    // generality, assume that R/G/B values are in the range 0..1.)
    //
    // Let Max = max(R,G,B), Min = min(R,G,B), and Med be the median value. Then L =
    // (Max+Min)/2. If L is to remain constant, Max+Min must also remain constant.
    //
    // For H to remain constant, first, the (numerical) order of R/G/B (from
    // smallest to largest) must remain the same. Second, all the ratios
    // (R-G)/(Max-Min), (R-B)/(Max-Min), (G-B)/(Max-Min) must remain constant (of
    // course, if Max = Min, then S = 0 and no saturation change is well-defined,
    // since H is not well-defined).
    //
    // Let C_max be a colour with value Max, C_min be one with value Min, and C_med
    // the remaining colour. Increasing saturation (to the maximum) is accomplished
    // by increasing the value of C_max while simultaneously decreasing C_min and
    // changing C_med so that the ratios are maintained; for the latter, it suffices
    // to keep (C_med-C_min)/(C_max-C_min) constant (and equal to
    // (Med-Min)/(Max-Min)).

    // Line processor: H no-op, S decrease, L no-op.
    void LineProcHnopSdecLnop(const color_utils::HSL& hsl_shift,
        const SkPMColor* in,
        SkPMColor* out,
        int width)
    {
        DCHECK(hsl_shift.h < 0);
        DCHECK(hsl_shift.s >= 0 && hsl_shift.s <= 0.5 - HSLShift::epsilon);
        DCHECK(hsl_shift.l < 0 || fabs(hsl_shift.l - 0.5) < HSLShift::epsilon);

        const int32_t denom = 65536;
        int32_t s_numer = static_cast<int32_t>(hsl_shift.s * 2 * denom);
        for (int x = 0; x < width; x++) {
            int32_t a = static_cast<int32_t>(SkGetPackedA32(in[x]));
            int32_t r = static_cast<int32_t>(SkGetPackedR32(in[x]));
            int32_t g = static_cast<int32_t>(SkGetPackedG32(in[x]));
            int32_t b = static_cast<int32_t>(SkGetPackedB32(in[x]));

            int32_t vmax, vmin;
            if (r > g) { // This uses 3 compares rather than 4.
                vmax = std::max(r, b);
                vmin = std::min(g, b);
            } else {
                vmax = std::max(g, b);
                vmin = std::min(r, b);
            }

            // Use denom * L to avoid rounding.
            int32_t denom_l = (vmax + vmin) * (denom / 2);
            int32_t s_numer_l = (vmax + vmin) * s_numer / 2;

            r = (denom_l + r * s_numer - s_numer_l) / denom;
            g = (denom_l + g * s_numer - s_numer_l) / denom;
            b = (denom_l + b * s_numer - s_numer_l) / denom;
            out[x] = SkPackARGB32(a, r, g, b);
        }
    }

    // Line processor: H no-op, S decrease, L decrease.
    void LineProcHnopSdecLdec(const color_utils::HSL& hsl_shift,
        const SkPMColor* in,
        SkPMColor* out,
        int width)
    {
        DCHECK(hsl_shift.h < 0);
        DCHECK(hsl_shift.s >= 0 && hsl_shift.s <= 0.5 - HSLShift::epsilon);
        DCHECK(hsl_shift.l >= 0 && hsl_shift.l <= 0.5 - HSLShift::epsilon);

        // Can't be too big since we need room for denom*denom and a bit for sign.
        const int32_t denom = 1024;
        int32_t l_numer = static_cast<int32_t>(hsl_shift.l * 2 * denom);
        int32_t s_numer = static_cast<int32_t>(hsl_shift.s * 2 * denom);
        for (int x = 0; x < width; x++) {
            int32_t a = static_cast<int32_t>(SkGetPackedA32(in[x]));
            int32_t r = static_cast<int32_t>(SkGetPackedR32(in[x]));
            int32_t g = static_cast<int32_t>(SkGetPackedG32(in[x]));
            int32_t b = static_cast<int32_t>(SkGetPackedB32(in[x]));

            int32_t vmax, vmin;
            if (r > g) { // This uses 3 compares rather than 4.
                vmax = std::max(r, b);
                vmin = std::min(g, b);
            } else {
                vmax = std::max(g, b);
                vmin = std::min(r, b);
            }

            // Use denom * L to avoid rounding.
            int32_t denom_l = (vmax + vmin) * (denom / 2);
            int32_t s_numer_l = (vmax + vmin) * s_numer / 2;

            r = (denom_l + r * s_numer - s_numer_l) * l_numer / (denom * denom);
            g = (denom_l + g * s_numer - s_numer_l) * l_numer / (denom * denom);
            b = (denom_l + b * s_numer - s_numer_l) * l_numer / (denom * denom);
            out[x] = SkPackARGB32(a, r, g, b);
        }
    }

    // Line processor: H no-op, S decrease, L increase.
    void LineProcHnopSdecLinc(const color_utils::HSL& hsl_shift,
        const SkPMColor* in,
        SkPMColor* out,
        int width)
    {
        DCHECK(hsl_shift.h < 0);
        DCHECK(hsl_shift.s >= 0 && hsl_shift.s <= 0.5 - HSLShift::epsilon);
        DCHECK(hsl_shift.l >= 0.5 + HSLShift::epsilon && hsl_shift.l <= 1);

        // Can't be too big since we need room for denom*denom and a bit for sign.
        const int32_t denom = 1024;
        int32_t l_numer = static_cast<int32_t>((hsl_shift.l - 0.5) * 2 * denom);
        int32_t s_numer = static_cast<int32_t>(hsl_shift.s * 2 * denom);
        for (int x = 0; x < width; x++) {
            int32_t a = static_cast<int32_t>(SkGetPackedA32(in[x]));
            int32_t r = static_cast<int32_t>(SkGetPackedR32(in[x]));
            int32_t g = static_cast<int32_t>(SkGetPackedG32(in[x]));
            int32_t b = static_cast<int32_t>(SkGetPackedB32(in[x]));

            int32_t vmax, vmin;
            if (r > g) { // This uses 3 compares rather than 4.
                vmax = std::max(r, b);
                vmin = std::min(g, b);
            } else {
                vmax = std::max(g, b);
                vmin = std::min(r, b);
            }

            // Use denom * L to avoid rounding.
            int32_t denom_l = (vmax + vmin) * (denom / 2);
            int32_t s_numer_l = (vmax + vmin) * s_numer / 2;

            r = denom_l + r * s_numer - s_numer_l;
            g = denom_l + g * s_numer - s_numer_l;
            b = denom_l + b * s_numer - s_numer_l;

            r = (r * denom + (a * denom - r) * l_numer) / (denom * denom);
            g = (g * denom + (a * denom - g) * l_numer) / (denom * denom);
            b = (b * denom + (a * denom - b) * l_numer) / (denom * denom);
            out[x] = SkPackARGB32(a, r, g, b);
        }
    }

    const LineProcessor kLineProcessors[kNumHOps][kNumSOps][kNumLOps] = {
        { // H: kOpHNone
            {
                // S: kOpSNone
                LineProcCopy, // L: kOpLNone
                LineProcHnopSnopLdec, // L: kOpLDec
                LineProcHnopSnopLinc // L: kOpLInc
            },
            {
                // S: kOpSDec
                LineProcHnopSdecLnop, // L: kOpLNone
                LineProcHnopSdecLdec, // L: kOpLDec
                LineProcHnopSdecLinc // L: kOpLInc
            },
            {
                // S: kOpSInc
                LineProcDefault, // L: kOpLNone
                LineProcDefault, // L: kOpLDec
                LineProcDefault // L: kOpLInc
            } },
        { // H: kOpHShift
            {
                // S: kOpSNone
                LineProcDefault, // L: kOpLNone
                LineProcDefault, // L: kOpLDec
                LineProcDefault // L: kOpLInc
            },
            {
                // S: kOpSDec
                LineProcDefault, // L: kOpLNone
                LineProcDefault, // L: kOpLDec
                LineProcDefault // L: kOpLInc
            },
            {
                // S: kOpSInc
                LineProcDefault, // L: kOpLNone
                LineProcDefault, // L: kOpLDec
                LineProcDefault // L: kOpLInc
            } }
    };

} // namespace HSLShift
} // namespace

// static
SkBitmap SkBitmapOperations::CreateHSLShiftedBitmap(
    const SkBitmap& bitmap,
    const color_utils::HSL& hsl_shift)
{
    // Default to NOPs.
    HSLShift::OperationOnH H_op = HSLShift::kOpHNone;
    HSLShift::OperationOnS S_op = HSLShift::kOpSNone;
    HSLShift::OperationOnL L_op = HSLShift::kOpLNone;

    if (hsl_shift.h >= 0 && hsl_shift.h <= 1)
        H_op = HSLShift::kOpHShift;

    // Saturation shift: 0 -> fully desaturate, 0.5 -> NOP, 1 -> fully saturate.
    if (hsl_shift.s >= 0 && hsl_shift.s <= (0.5 - HSLShift::epsilon))
        S_op = HSLShift::kOpSDec;
    else if (hsl_shift.s >= (0.5 + HSLShift::epsilon))
        S_op = HSLShift::kOpSInc;

    // Lightness shift: 0 -> black, 0.5 -> NOP, 1 -> white.
    if (hsl_shift.l >= 0 && hsl_shift.l <= (0.5 - HSLShift::epsilon))
        L_op = HSLShift::kOpLDec;
    else if (hsl_shift.l >= (0.5 + HSLShift::epsilon))
        L_op = HSLShift::kOpLInc;

    HSLShift::LineProcessor line_proc = HSLShift::kLineProcessors[H_op][S_op][L_op];

    DCHECK(bitmap.empty() == false);
    DCHECK(bitmap.colorType() == kN32_SkColorType);

    SkBitmap shifted;
    shifted.allocN32Pixels(bitmap.width(), bitmap.height());

    SkAutoLockPixels lock_bitmap(bitmap);
    SkAutoLockPixels lock_shifted(shifted);

    // Loop through the pixels of the original bitmap.
    for (int y = 0; y < bitmap.height(); ++y) {
        SkPMColor* pixels = bitmap.getAddr32(0, y);
        SkPMColor* tinted_pixels = shifted.getAddr32(0, y);

        (*line_proc)(hsl_shift, pixels, tinted_pixels, bitmap.width());
    }

    return shifted;
}

// static
SkBitmap SkBitmapOperations::CreateTiledBitmap(const SkBitmap& source,
    int src_x, int src_y,
    int dst_w, int dst_h)
{
    DCHECK(source.colorType() == kN32_SkColorType);

    SkBitmap cropped;
    cropped.allocN32Pixels(dst_w, dst_h);

    SkAutoLockPixels lock_source(source);
    SkAutoLockPixels lock_cropped(cropped);

    // Loop through the pixels of the original bitmap.
    for (int y = 0; y < dst_h; ++y) {
        int y_pix = (src_y + y) % source.height();
        while (y_pix < 0)
            y_pix += source.height();

        uint32_t* source_row = source.getAddr32(0, y_pix);
        uint32_t* dst_row = cropped.getAddr32(0, y);

        for (int x = 0; x < dst_w; ++x) {
            int x_pix = (src_x + x) % source.width();
            while (x_pix < 0)
                x_pix += source.width();

            dst_row[x] = source_row[x_pix];
        }
    }

    return cropped;
}

// static
SkBitmap SkBitmapOperations::DownsampleByTwoUntilSize(const SkBitmap& bitmap,
    int min_w, int min_h)
{
    if ((bitmap.width() <= min_w) || (bitmap.height() <= min_h) || (min_w < 0) || (min_h < 0))
        return bitmap;

    // Since bitmaps are refcounted, this copy will be fast.
    SkBitmap current = bitmap;
    while ((current.width() >= min_w * 2) && (current.height() >= min_h * 2) && (current.width() > 1) && (current.height() > 1))
        current = DownsampleByTwo(current);
    return current;
}

// static
SkBitmap SkBitmapOperations::DownsampleByTwo(const SkBitmap& bitmap)
{
    // Handle the nop case.
    if ((bitmap.width() <= 1) || (bitmap.height() <= 1))
        return bitmap;

    SkBitmap result;
    result.allocN32Pixels((bitmap.width() + 1) / 2, (bitmap.height() + 1) / 2);

    SkAutoLockPixels lock(bitmap);

    const int resultLastX = result.width() - 1;
    const int srcLastX = bitmap.width() - 1;

    for (int dest_y = 0; dest_y < result.height(); ++dest_y) {
        const int src_y = dest_y << 1;
        const SkPMColor* SK_RESTRICT cur_src0 = bitmap.getAddr32(0, src_y);
        const SkPMColor* SK_RESTRICT cur_src1 = cur_src0;
        if (src_y + 1 < bitmap.height())
            cur_src1 = bitmap.getAddr32(0, src_y + 1);

        SkPMColor* SK_RESTRICT cur_dst = result.getAddr32(0, dest_y);

        for (int dest_x = 0; dest_x <= resultLastX; ++dest_x) {
            // This code is based on downsampleby2_proc32 in SkBitmap.cpp. It is very
            // clever in that it does two channels at once: alpha and green ("ag")
            // and red and blue ("rb"). Each channel gets averaged across 4 pixels
            // to get the result.
            int bump_x = (dest_x << 1) < srcLastX;
            SkPMColor tmp, ag, rb;

            // Top left pixel of the 2x2 block.
            tmp = cur_src0[0];
            ag = (tmp >> 8) & 0xFF00FF;
            rb = tmp & 0xFF00FF;

            // Top right pixel of the 2x2 block.
            tmp = cur_src0[bump_x];
            ag += (tmp >> 8) & 0xFF00FF;
            rb += tmp & 0xFF00FF;

            // Bottom left pixel of the 2x2 block.
            tmp = cur_src1[0];
            ag += (tmp >> 8) & 0xFF00FF;
            rb += tmp & 0xFF00FF;

            // Bottom right pixel of the 2x2 block.
            tmp = cur_src1[bump_x];
            ag += (tmp >> 8) & 0xFF00FF;
            rb += tmp & 0xFF00FF;

            // Put the channels back together, dividing each by 4 to get the average.
            // |ag| has the alpha and green channels shifted right by 8 bits from
            // there they should end up, so shifting left by 6 gives them in the
            // correct position divided by 4.
            *cur_dst++ = ((rb >> 2) & 0xFF00FF) | ((ag << 6) & 0xFF00FF00);

            cur_src0 += 2;
            cur_src1 += 2;
        }
    }

    return result;
}

// static
SkBitmap SkBitmapOperations::UnPreMultiply(const SkBitmap& bitmap)
{
    if (bitmap.isNull())
        return bitmap;
    if (bitmap.isOpaque())
        return bitmap;

    const SkImageInfo& info = bitmap.info();
    SkImageInfo opaque_info = SkImageInfo::Make(info.width(), info.height(), info.colorType(),
        kOpaque_SkAlphaType, info.profileType());
    SkBitmap opaque_bitmap;
    opaque_bitmap.allocPixels(opaque_info);

    {
        SkAutoLockPixels bitmap_lock(bitmap);
        SkAutoLockPixels opaque_bitmap_lock(opaque_bitmap);
        for (int y = 0; y < opaque_bitmap.height(); y++) {
            for (int x = 0; x < opaque_bitmap.width(); x++) {
                uint32_t src_pixel = *bitmap.getAddr32(x, y);
                uint32_t* dst_pixel = opaque_bitmap.getAddr32(x, y);
                SkColor unmultiplied = SkUnPreMultiply::PMColorToColor(src_pixel);
                *dst_pixel = unmultiplied;
            }
        }
    }

    return opaque_bitmap;
}

// static
SkBitmap SkBitmapOperations::CreateTransposedBitmap(const SkBitmap& image)
{
    DCHECK(image.colorType() == kN32_SkColorType);

    SkBitmap transposed;
    transposed.allocN32Pixels(image.height(), image.width());

    SkAutoLockPixels lock_image(image);
    SkAutoLockPixels lock_transposed(transposed);

    for (int y = 0; y < image.height(); ++y) {
        uint32_t* image_row = image.getAddr32(0, y);
        for (int x = 0; x < image.width(); ++x) {
            uint32_t* dst = transposed.getAddr32(y, x);
            *dst = image_row[x];
        }
    }

    return transposed;
}

// static
SkBitmap SkBitmapOperations::CreateColorMask(const SkBitmap& bitmap,
    SkColor c)
{
    DCHECK(bitmap.colorType() == kN32_SkColorType);

    SkBitmap color_mask;
    color_mask.allocN32Pixels(bitmap.width(), bitmap.height());
    color_mask.eraseARGB(0, 0, 0, 0);

    SkCanvas canvas(color_mask);

    skia::RefPtr<SkColorFilter> color_filter = skia::AdoptRef(
        SkColorFilter::CreateModeFilter(c, SkXfermode::kSrcIn_Mode));
    SkPaint paint;
    paint.setColorFilter(color_filter.get());
    canvas.drawBitmap(bitmap, SkIntToScalar(0), SkIntToScalar(0), &paint);
    return color_mask;
}

// static
SkBitmap SkBitmapOperations::CreateDropShadow(
    const SkBitmap& bitmap,
    const gfx::ShadowValues& shadows)
{
    DCHECK(bitmap.colorType() == kN32_SkColorType);

    // Shadow margin insets are negative values because they grow outside.
    // Negate them here as grow direction is not important and only pixel value
    // is of interest here.
    gfx::Insets shadow_margin = -gfx::ShadowValue::GetMargin(shadows);

    SkBitmap image_with_shadow;
    image_with_shadow.allocN32Pixels(bitmap.width() + shadow_margin.width(),
        bitmap.height() + shadow_margin.height());
    image_with_shadow.eraseARGB(0, 0, 0, 0);

    SkCanvas canvas(image_with_shadow);
    canvas.translate(SkIntToScalar(shadow_margin.left()),
        SkIntToScalar(shadow_margin.top()));

    SkPaint paint;
    for (size_t i = 0; i < shadows.size(); ++i) {
        const gfx::ShadowValue& shadow = shadows[i];
        SkBitmap shadow_image = SkBitmapOperations::CreateColorMask(bitmap,
            shadow.color());

        // The blur is halved to produce a shadow that correctly fits within the
        // |shadow_margin|.
        SkScalar sigma = SkDoubleToScalar(shadow.blur() / 2);
        skia::RefPtr<SkImageFilter> filter = skia::AdoptRef(SkBlurImageFilter::Create(sigma, sigma));
        paint.setImageFilter(filter.get());

        canvas.saveLayer(0, &paint);
        canvas.drawBitmap(shadow_image,
            SkIntToScalar(shadow.x()),
            SkIntToScalar(shadow.y()));
        canvas.restore();
    }

    canvas.drawBitmap(bitmap, SkIntToScalar(0), SkIntToScalar(0));
    return image_with_shadow;
}

// static
SkBitmap SkBitmapOperations::Rotate(const SkBitmap& source,
    RotationAmount rotation)
{
    // SkCanvas::drawBitmap() fails silently with unpremultiplied SkBitmap.
    DCHECK_NE(source.info().alphaType(), kUnpremul_SkAlphaType);

    SkBitmap result;
    SkScalar angle = SkFloatToScalar(0.0f);

    switch (rotation) {
    case ROTATION_90_CW:
        angle = SkFloatToScalar(90.0f);
        result.allocN32Pixels(source.height(), source.width());
        break;
    case ROTATION_180_CW:
        angle = SkFloatToScalar(180.0f);
        result.allocN32Pixels(source.width(), source.height());
        break;
    case ROTATION_270_CW:
        angle = SkFloatToScalar(270.0f);
        result.allocN32Pixels(source.height(), source.width());
        break;
    }

    SkCanvas canvas(result);
    canvas.clear(SkColorSetARGB(0, 0, 0, 0));

    canvas.translate(SkFloatToScalar(result.width() * 0.5f),
        SkFloatToScalar(result.height() * 0.5f));
    canvas.rotate(angle);
    canvas.translate(-SkFloatToScalar(source.width() * 0.5f),
        -SkFloatToScalar(source.height() * 0.5f));
    canvas.drawBitmap(source, 0, 0);
    canvas.flush();

    return result;
}
